Signal wave transmission system



Jgly s, 1941. I l K H, DAVIS 2,248,746r

SIGNAL WAVE TRANSMISSION SYSTEM ATTORNEY July 8,1941.,A K. H. DAvrs l2,248,746

SIGNAL WAVE TRANSMISSION SYSTEM- BV VM A7' TORNEV Patented July 8, 1941SGNAL WAVE TRANSMISSION SYSTEM Kingsbury H. Davis, Closter, N. J.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application March 22, 1940, Serial No. 325,341

6 Claims.

This invention relates to signal Wave transmission systems and moreparticularly to waveoperated switching circuits for controlling signaltransmission in such systems.

The invention, in particular, refers to two-way carrier wave signalingsystems, for example, radio telephone systems in which, in order toovercome fading and other variations that affect the amplitude of thereceived signal, the gain of the receiver is automatically controlled bythe magnitude of the received carrier wave component or by other controlenergy.

In systems of the type to which reference has been made it is oftendesirable to so condition the transmitting and receiving circuits thatsinging does not take place even if the receiving channel is notlperfectly conjugate to the high power transmitting channel. Unless thisis done, the local transmitter may subject its local receiver to anoverpowering side-tone. It is also desirable to prevent theretransmission over the transmitting channel of line noise duringnonsignaling intervals or of line echoes of th-e received signal duringperiods when the receiver is functioning. These and similar purposes maybe fulfilled by apparatus of the so-called vodas type (voice-operateddetector anti-singing).

An object of the invention is to improve the coordination between theterminal apparatus of two-way radio telephone equipment by renderingmore certain the proper actuation of the circuit conditioning devices.

Another object of the invention is to insure that in two-way signalingsystems employing tone control the tone shall not be adversely affectedrelative to noise or interfering impulses because of the action ofautomatic volume control apparatus.

An additional object is to reduce the effects of selective fading on thecircuit controls of two-Way signal wave transmission circuits.

Since Vodas apparatus should respond to incoming or outgoing speechwaves but not to noise there is the problem of insuring against falseoperation which may occur under severe conditions of fading or otherconditions during which the available incoming speech level may fallWithin the range of the maximum noise level.

In accordance with one embodiment of the invention as applied toy radiotelephone systems, speech currents originating at one terminal of thetwo-way system cause a tone to be applied as a modulation of theoutgoing carrier wave. The tone, which is preferably of a frequencyadjacent to one limit of the essential speech frequency range, servesaft-er detection at the remote radio receiver to condition the receivingcircuit of which the remote radio receiver forms a part and, also, todisable the transmitting circuit'which is vassociated with the remoteradio receiver "at the remote terminal. The tone so derived from theoutput current at the remote receiver reflects in its magnitude theeffects of the automatic volume control action of the radio receiver andsince noise will be varied in amplitude by adjustments of the automaticvolume control in the radio receiver, the minimum amplitude of the tonerelative to that of the maximum amplitude of interfering noise may, attimes, have been reduced. The invention overcomes this effect bysubjecting the tone to a gain control which is the reverse of thatoccui-ring in the receiver so that the tone as utilized for operation ofthe circuit conditioning apparatus is free from the limitations as tomagnitude that the automatic volume control might tend to impose.

vfn the drawings, Fig. 1 illustrates in schematic diagram the elementsof one terminal of a two- Way radio telephone system with its associatedland line comprising one embodiment of the invention.

Fig. 2 illustrates a similar system provided with an auxiliary circuitconditioning apparatus.

Fig. 3 illustrates a modification of the system of Fig. 1 in which thereverse gain control apparatus is included as an immediate adjunct ofthe two-way transmission line rather than of the Vradio receiver.

Fig. 4 illustrates a modification of the system of Fig. 3 in whichauxiliary circuit conditioning apparatus is provided.

The system is illustrated in the drawings not by actual circuit diagrambut instead by a single' line layout, each l'ine indicating a two-wiretransmission path. A break in the path represented by separatedarrow-heads indicates that the path is normally disabled at that pointand an arrow pointing toward such a break from a control device(rectifier, which ordinarily signies a thermionic amplier and detectorcombination) indicates that the path will be made operative by operationof the control device. A closed circuit condition represented bycontacting arrow-heads indicates that the path is operative at thatpoint and an arrow pointed towards the contacting arrow-heads fromacontrol device (rectifier) indicates that the path Will be disabled atthat point by operation of thev control device, An arrow pointing fromone box representing a control device to another box representinganother apparatus indicates that The invention may be readily understoodfrom Va consideration of the operation of the circuit in which it isembodied. Referring to Fig. 1

' ting suppressor 5 Vto a delay network 6.

which illustrates a two-way radio terminal station associated with atwo-way conductive transmission system, the two-Way telephone or othercommunication conductive system I is associated through the hybrid coil2 with an outgoing channel 3 connected to the radio transmittingequipment and an incoming channel 4 connected with radio receivingequipment. The broken line A-A serves to indicate the division betweenthe line terminal equipment on the one hand and the radio equipment onthe other. They radio receiving apparatus and the radio transmittingapparatus may -both or either be located at sites remote from the lineterminal equipment. Speech currents arriving over the line-I aretransferred by the hybrid coil 2 to channel 3 and vare repeated by anaudio frequency repeater 24 of well-known thermionic type. After anamplication which may yield a gain as high as 40 to 45 decibels, thespeech currents pass by way of the normally closed circuit of transmit-The transmitting suppressor 5 may be of the well- 'knownV type ofcontrol apparatus disclosed 'in Fig. 2 of U. S. Patent 2,098,286, issuedNovember 9, V193'? to O. R. Garfield. Control apparatus of thischaract-er ordinarily involves a rectifier. responsive to speech orother currents to produce a resulting rectified current electromotiveforce which is then applied to the input circuit of an amplifier Iin thespace path of which is included an electromagnetic relay, the contactsof which are associated with the circuit to be controlled.

Speech or other signal electromotive forces impressed upon the delaynetwork 6 are simultaneously impressed upon the input terminals of arectifier I the output terminals of which are associated withtransmitting singing suppressor 8 and control device 9. It will beunderstood that the devices 8 and 9 each involve normally open circuitapparatus provided with electromagnetic or other equivalent circuitclosers operated by, the rectified currents from rectifier 1. Operationof the device 8 to close its circuit permits the speech currents to betransmitted over circuit I0 to a radio transmitter II at which they areemployed to modulate an outgoing carrier wave which is thereafterradiated from transmitting antenna I2. The delay network 6 serves todelay transmission of the speech currents sufliciently to enable thedevice 8 to establish an outgoing path for the speech. A delay of theorder of 20 milliseconds affords sufficient time for the mechanicalcircuit conditioning equipment to operate.

In order to effect certain circuit conditioning operations at the remotestation, it is desirable to transmit as a modulation of the outgoingcarrier wave a tone frequency. For this purpose a source I3 is used toproduce a tone lying at one fringe of the essential signal frequencyrange. In what follows it will be assumed that the signal waves 'arespeech waves and that the frequency of the tone is 150 cycles althoughit might equally well be 3500 cycles. The source I3 of tone current isconnected to the radio transmitter II by way of the control apparatus 9and a tone band-pass filter I-I of suiiiciently narrow transmissionrange to freely transmit the'tone but to exclude other currents in thespeech frequency range. The radio transmitter may be of the type inwhich the unmodulated carrier wave is transmitted whenever thetransmitting apparatus is Venergized or it may be of the type in whichthe unmodulated component isnot transmitted except during azien/16 Ysignal transmission or is partially suppressed even during carrier wavemodulation by the signal. Assuming that the transmitter is energized,the antenna I2 will radiate carrier waves modulated in accordance withboth speech and tone whenever speech currents are received from theyline I,

In order tol guard the outgoing transmitting circuit against undesiredoperation of its circuit conditioning devices because of energy receivedover the radio receiving channel, there is also connected to the outputof the rectifier I a receiving suppressor I6 which is in series with theincoming channel and serves to disable that channel during outgoingmessage intervals.

Inasmuch as the remote radio apparatus cooperating with that of Fig. 1may be similar in all respects thereto, except that the transmittingcarrier wave of one is the receiving carrier wave of the other, it willsuffice to trace an incoming call at the radio receiver associated withthe terminals of Fig. 1. Radio receiving antenna II is, of course, tunedto the same carrier frequency as the remote transmitting antenna fromwhich it receives modulated carrier waves. It is preferably but notnecessarily tuned to a different frequency from that of its localtransmitter. Incoming carrier Waves modulated in accordance with signalsand with control tone are impressed by antenna I1 upon the radioreceiver which may be of any well-known type involving the usualautomatic volume control apparatus to overcome the effects of failure.Speech frequency current and tone current resulting from thedemodulatingA operation of the radio receiver I8 upon incoming carrierwaves are impressed from its output upon a tone band exclusion filter I9which serves to transmit freely currents of all the essential speech orother signal frequencies with the exception of a very narrow bandincluding the tone frequency, Such a lter may be of the .high-pass typeif the tone is at the lower edge of the signal frequency range, of thelow-pass type if the tone is at the upper edge of the essentialfrequency range, or it may be of the so-called suppression type whichsuppresses a narrow band. The speech signal .currents in the output ofdevice I9 are transmitted by way of the normally closed circuit ofreceiving channel suppressor IS to a delay network 20 which serves todelay the signal current sufficiently to enable the operation of theelectromechanical control of the normally opencircuited receivingchannel singing suppressor 2|. In order to remove the normally disabledcondition of the path at device 2I there is connected to the output ofthe receiving channel suppressor I6 in parallel to the input terminalsof the delay network 20 a narrow band-pass filter 22, thecharacteristics of which are such as to permit transmission of a narrowband of frequencies in the neighborhood of the tone, possibly cycleswide. A rectifier-amplifier 23 connected to the output of band-passfilter 22 applies rectified electromotive force to operate devices 5 and2| simultaneously disabling the outgoing circuit at 5 and completing theincoming circuit at 2I. The speech currents delayed in the network 20may therefore traverse device 2I and are repeated by audio repeater I5from the output circuit of which they are transferred by hybrid coil 2to the line I. Since it is desired that the control of devices 5 and 2Ibe effective only in response to tone current and not in response tonoise current, it is desirable that the ratio of tone current .to noiseenergy be maintained as high as possible, in order to prevent operationof devices and 2| `at undesired moments because of receipt of incomingcarrier waves having noise modulation in the region of the tonefrequency. It transpires, however, that an automatic volume controlapparatus may at timesl affect the -ratio of tone to noise unfavorably.This is because of the fact that selective fading as between varioustransmitted frequencies may occur thus at times causing the tonefrequency to fade to a greater extent thandoes the carrier wave uponlwhich the automatic volume control depends. Under such lconditions theautomatic volume control may actually reduce the amplification of thereceiver when the incoming carrier wave component has increased and somay reduce the resulting control tone at a time when it has alreadyfaded. The net result of this effect is an increase in the fading rangeof the, control tone. The undesired discrimination against the tonefrequency could be prevented if the tone component were not subject tothe automatic volume control. However, it would be difficult andexpensive to separate the tone control component from the signalcomponent at the frequency of the incoming carrier wave. An importantfeature of the present invention relates to the method by which this iseffectively accomplishedI without resort to separation at the radiofrequency. Connected to the output of the radio receiver and in parallelto the input terminals of tone band exclusion filter I9 is a toneband-pass filter y25. The transmission range of this lter may be made ofthe same order as that of the other tone bandpass filters in thecircuit; for example, 100 cycles wide. The tone selected from the outputof the radio receiver I8 by the filter 25 is then applied to a reversegain control apparatus 26. Apparatus 2E serves to variably amplify thetone current impressedvthereon but in a fashion reverse in manner tothat of the automatic volume control apparatus of the radio receiver I8.For this purpose there is supplied to it over the path 2I from theautomatic volume control apparatus of .the radio receiver a potentialwhich is the same as that used for the automatic volume controloperation of the radio receiver. This control potential is applied bythe circuit 2'I to the reverse gain control apparatus 26 with suchpolarity as to cause an increase in gain to the tone which is theconverse of the decrease in gain received at the radio receiver and tosupply a decrease in gain at the reverse gain control apparatus which isthe converse of an increase in gain applied to the radio receiver I8. Itfollows that the tone output from the reverse gain control apparatus 26in effect has been unaffected by the automatic volume control apparatusof the radio receiver I8 since the amplitude variations to which it wassubjected as a radio frequency component in traversing the radioreceiver have been counteracted by the amplitude variations to which ithas been subjected in the reverse gain control apparatus as a tonecontrol frequency component. Tone control current in the output of thedevice 26| will be transmitted through the normally closed contacts ofthe receiving channel suppressor I6, the band-pass filter 22 torectifier 23, where it will be rectified and caused to disable theoutgoing channels at 5 and to make effective the transmission path ofthe incoming channel at 2I. Accordingly, any unbalance at the hybridcoil. 2 will not result in causing received currents to recirculatethrough the outgoing channel 3 to the radio transmitter II sincethe'outgoingchannel is interrupted at 5. y

Fig. 2 shows a modification of the system of Fig. l provided withadditional auxiliary control apparatus which is not essential wherecontrol tone is used but may be helpful to enable the control functionto be carried on in response to received speech when the tone has fadedbadly. Apparatus of Fig. 2 which functions in precisely the same manneras that of Fig. 1 is designated by the same reference characters. In theoperation o-f the system of Fig. 2 speech currents originatingrin line Iare passed by the hybrid coil to the outgoing circuit 3 over an audiorepeater 24 from` which they are passed to a transmitting singingsuppressor 29 which corresponds generally to device 5 of Fig. 1 exceptthat it has two control points in tandem. One of these control points iscontrolled by the amplifier-rectifier 23 just as in the Icase of Fig. 1.The other is controlled by the output of an auxiliary rectifier 3Uconnected to the receiving channel between the control point I6 and thedelay network 2IJ. Speech currents traversing the outgoing channel aresupplied to the delay network 6 and the rectifier 'I which serves tomake operative the paths at 8 and 9 for the speech currents and tonecurrents respectively in order that these currents may be impressed overline I0 upon the radio transmitter I I and radiated from the antenna I2as modulations of the outgoing carrier wave.' Incoming carrier wavesmodulated in accordance with speech and control tone received 4overantenna I'I are demodulated by radio receiver I8 and the speech and tonecontrol currents are separated by the lters I9 and 25, respectively,'asin the system of Fig. 1. After the reverse gain control operation uponthe tone current the speech and tone control currents traverse thecontrol point I6 and are applied to both the rectifier 3U and theband-pass lter 22 in parallel. If the amplitude of the tone control.current is suflicient the operation of the system is exactly the sameas in that of Fig. 1, the outgoing channel being disabled at the firstset of terminals of the device 29 and the incoming channel, which isnormally open at the contact of the receiving singing suppressor 3I,being closed. It will be noted that the device 3I is provided with twocontrols, the operation of either of which serves to close it.Similarly. the device 29 is provided with two controls, the operation ofeither of which serves to disable its transmitting path. The rectifier30 is provided with a tuned input circuit preferably tuned to about 1300cycles and hence is sensitive to speech but insensitive to tone. If,therefore, the tone has considerably faded but the speech has not, thedevice 30 will serve to operate the controls 29 and 3|.

Fig. 3 illustrates a system in which the reverse gain control apparatusfor the tone is located at the terminal station associated with thetwo-way transmission line I rather than at the remote radio receivingstation as in Figs. 1 and 2. Apparatus which is similar in constructionand function to that of Fig. 1 is similarly designated and willVaccordingly require no explanation. There is provided at the receivingchannel in parallel to the input circuit of the delay network 20 aband-pass filter 33 which transmits a narrow band in the neighborhood ofthe tone and of the order of cycles in width. Accordingly, the bandfilter 33 accepts and transmits only the tone and noise or staticimpulses in the frequency region of the tone. Connected to the outputterminals o'f the band-pass filter 33 is a reverse gain control device34 similar in its Vstr1 1ctureand operation to the device 26 of Figs. 1and 2 but differing in that it. does not respond to the same controlpotential as does the automatic volume control of the radioj receiverI8. The output control. toneV and static impulses from the reverse gaincontrol device 34 lare impressed upon a path 35 in which is a tone bandexclusion filter 36 which serves to transmit the static impulses but toexclude all components in a very narrow .band of perhaps 10 cycles inwidth including the control tone itself. Accordingly, there is impressedby the device 36 upon the rectifier 31 only static impulses in theregion ofthe control tone frequency. These are rectied and caused toyield a control potential which is applied to the reverse gain controlVapparatus to control the gain. In general, the static and other noisereceived at the radio receiving station may be assumed to be fairlyconstant. However, in the course ofthe radio receiving operation, allreceived energy, including that of thel static, is subjected to theyvariable amplification imposed by the automatic volume controlapparatus of the radio receiver I8. This means that both the controltone and the rincoming static of substantially constant energy level arelikewise varied by the automatic volume control apparatus. Since thefilter 36 selects the static energy, rectiiies it and controls thereverse gain control apparatus 34 in accordanceI with the rectiedpotential to maintain the substantially constant level of static at theoutl'put ofthedevice 34 in the manner of an automatic volume controlapparatus, the control tone traversing the device 34 is similarly variedin level and restored to its initial intensity as received at the radioreceiver I8. A selective ampliiier-rectilier 38 tuned to select thecontrol tone'and to amplify and rectify it in order to derive a controlpotential therefrom serves to ycontrol the apparatus and 2I in themanner which has previously been explained in connection withthe'systems of Figs. 1 and 2. In other respects, the operation of thesystem of Fig.,3 is identical with that of Fig. 1.

The system of Fig.l 4 resembles that of Fig. 3 in that the reverse gaincontrol apparatus for the tone' is located at the terminal station forthe transmission line `I and is operated in response to rectified staticenergy of the frequency region of the control tone itself. In order,however, to insure that there will be effective control of the incomingand outgoing channels at times when the control tone may for somereasonhave been subjected to such severe fading action as not to be usable, anauxiliary control comprising the rectie`r30 and the double control units29 and 3| are added as in the system of Fig. 2. The operation of thesystem of Fig. 4 will therefore be evident without further explanation..

What is claimed is:

'1. The method which comprises modulating a carrier wave simultaneouslyby two different elecvtromotive forces, transmitting the modulatedceiving and detecting operation While controlling the volume of thedetected energy in accordance with the strength of the received carrierwave, selecting one of the two different modulations from the detectedwave and subjecting the Vselected modulation to a gain control operationwhichv is the reverse of that experienced in the detecting operation byvirtue of the automatic volume control whereby the selected modulationremains relatively unaffected by the volume control operation. I Y Y f2. A wave transmission system comprising a source of carrier waves,means for simultaneously modulating the Waves by signals and by acontrol tone, means for transmitting energy of the modulated Waves to aremote point, means at the remote point for receiving and detecting themodulated carrier waves, said means including an automatic volumecontrol, means for selecting the control tone from the detected wave,and means Afor subjecting the selected Vcontrol tone to an automaticvolume control operation which is the reverse of that Yexperienced inthe receiving means to eliminate the effect of automatic volume control.l

3. A receiver for modulated waves comprising a demodulator upon whichreceived modulated waves may be impressed, means forselecting from theoutput of the' demodulator waves'of a desired frequency, meansassociated with the receiver for deriving from the received Waves anautomatic volume'control potential which has at all times such intensityand frequency character.- istics as to enable compensation for fadingwhich has been experienced by modulated Waves as received, and means forutilizing the control potential for controlling the amplitude of theselected demodulated waves to cause them to experience an attenuationcorresponding to the attenuation experienced by the modulated wavesbefore they were received.

4. A carrier wave system comprising means for transmitting carrier wavesmodulated in accordance with signal waves, means for modulating thevsame carrier wave in accordance with control tones, a high frequencyreceiver for receiving the modulated carrier waves, said high frequencyreceiver comprising means to demodulate the carrier waves to reproducethe modulating signal waves and control tones, the receiver alsoincluding automatic volume rcontrol apparatus, and means for removingthe effect of the automatic volume control upon amplitude of the controltones yielded by the receiver. Y

5. The method of two-way signaling by carrier waves modulated inaccordance with signals which comprises modulating outgoing carrierWaves by control tone, receiving and demodulating incoming carrier wavesto repro-duce the control tone while causing the volume to beautomatically controlled in such manner as to compensate for variationsin attenuation during transmission, selecting the control tone andthereafter subjecting it to a reverse gain control operation whichremoves the effect of any automatic volume control operation to whichitv Was subjected during reception and demodulation of the incomingcarrier wave.

6. A carrier waveV system comprising a carrier wave source, means formodulating carrier waves supplied by the source of speechsignals and by'control tone of a neighboring frequency, areceiver for demodulating themodulated wave and for maintaining at substantially constant level thevolume of the speech signals derived therefrom, and means whereby thelcontrol tone derived from the receiver is freed from the volumeregulation to which the derived speech signals are subjected so that thefading range of the control tone may not be Yincreased to such a pointas to cause the control tone tobeoverridden by noise.l

KINGSBURY H. DAVIS.

